Spacecraft perform various maneuvers after they are launched into space and once they are on-station in an intended orbit. For example, after a spacecraft is launched into a low orbit, it may be required to raise the spacecraft to a higher (e.g., geosynchronous) orbit by firing the spacecraft's main thruster. This type of maneuver is known as an orbit-raising maneuver . Also by example, after the spacecraft is on-station in a selected orbit, various forces (e.g., solar and/or other environmental disturbance torques, such as magnetic torques) may act on the spacecraft and cause the spacecraft to drift away from its selected orbit into another, incorrect orbit. Thus, periodic (e.g., daily, weekly, or monthly) orbital maneuvers are often required to return the spacecraft to the correct orbit. These types of maneuvers are known as station-keeping maneuvers.
During the performance of each type of maneuver, the precise control of the spacecraft's attitude to orient the spacecraft's payload, such as communication or imaging hardware, to a preselected planetary location and/or to correctly orient the spacecraft's thrust vector is essential. Thus, spacecraft are typically equipped with closed-loop control systems which enable the attitude of the spacecraft to be controlled within pre-established deadband limits. Such control systems often employ spacecraft thrusters for selectively producing torques on the spacecraft for correcting the spacecraft attitude. By example, during orbit-raising maneuvers, attitude control can be maintained by activating selected ones of the spacecraft's thrusters to create a desired torque in order to correct the spacecraft's attitude.
The following commonly assigned U.S. Patents are illustrative of various approaches to providing spacecraft attitude control: U.S. Pat. No. 5,459,669, Control System And Method For Spacecraft Attitude Control, to Adsit et al.; U.S. Pat. No. 5,400,252, Spacecraft East/West Orbit Control During A North Or South Stationkeeping Maneuver, to Kazimi et al.; U.S. Pat. No. 5,349,532, Spacecraft Attitude Control And Momentum Unloading Using Gimballed And Throttled Thrusters, to Tilley et al.; and U.S. Patent No. 5,222,023, Compensated Transition For Spacecraft Attitude Control, to Liu et al.
Reference can also be had to U.S. Pat. No. 5,184,790, Two-Axis Attitude Correction For Orbit Inclination, to Fowell; U.S. Pat. No. 4,931,942, Transition Control System For Spacecraft Attitude Control, to Garg et al.; U.S. Pat. No. 4,848,706, Spacecraft Attitude Control Using Coupled Thrusters, Garg et al.; U.S. Pat. No. 4,767,084, Autonomous Stationkeeping For Three-Axis Stabilized Spacecraft, to Chan et al.; U.S. Pat. No. 4,759,517, Station-Keeping Using Solar Sailing, to Clark; U.S. Pat. No. 4,599,697, Digital PWPF Three Axis Spacecraft Attitude Control, to Chan et al.; U.S. Pat. No. 4,521,855, Electronic On-Orbit Roll/Yaw Satellite Control, to Lehner et al.; U.S. Pat. No. 4,489,383, Closed-Loop Magnetic Roll/Yaw Control System For High Inclination Orbit Satellites, to Schmidt, Jr.; and U.S. Pat. No. 4,084,772, Roll/Yaw Body Steering For Momentum Biased Spacecraft, to Muhlfelder.
Reference is also made to a publication entitled "Attitude Stabilization of Flexible Spacecraft During Stationkeeping Maneuvers", Bong Wie et al., J. Guidance, Vol. 7, No. 4, pgs. 430-436, July-August 1984.
Reference can also be had to European Patent Application No. 0 499 815 Al, Triaxially Stabilized Satellite Provided with Electric Propulsors for Orbital Maneuvering and Attitude Control, to Mazzini.
A typical geosynchronous satellite is designed to minimize solar torque imbalance. This is typically accomplished with symmetric solar array design, with the solar arrays 26 being located on the north and south side of the spacecraft (FIG. 5A), or in a configuration with the solar arrays 26 on the south side, balanced by a solar sail 29 on the north side (FIG. 5B). These appendages extend from a spacecraft bus 11. Residual solar and environmental disturbance torques are stored in momentum wheels that are then unloaded periodically using high thrust thrusters, magnetic torquers, trim tabs, or solar panel angle adjustments.
It can be appreciated that the inclusion of conventional momentum wheels and wheel unloading devices will typically increase the mass, complexity and cost of the spacecraft.